Ascent Rate is Key  Determining the Ascent Rate is a key part of using this app.  The calculations are based off of this.  If you do not have the correct Ascent Rate then the calculations will be off. 

The app has a built in Ascent Rate calculator (Helium Only) which is based off the formula published by P. Lowell and J. Reynolds (The Rise and Fall of Pibals. Balloon Life, page 14, March 1998).

You can also calculate the ascent rate using other methods of weighing the payload and using other formulas.  This is useful if you want to calculate the Ascent Rate for a pibal with a light attached as that will affect that rate.

The other key piece is to always inflate the Pibal to the same size every time.  What we use is a piece of webbing that is secured at a specific diameter.  We place the balloon inside the webbing and inflate until it is the size of the webbing.  That guarantees that we inflate to the same size every time.  We have 3 different ones for 11, 14 and 16 inch Pibals.  We realized that 12 inch balloons were difficult to inflate to 12 inches so we only inflate those to 11 inches.